I. Field of the Invention
The present invention concerns improvements relating to the connection between a conical shank and conical receptacle in general, and more specifically to an improved arrangement for connecting tool holders and spindles in machine tools.
II. Background of the Invention
Conical connections are widely used for precise collocation of parts in mechanical designs. A conical shank commonly has a counterpart in and is fitted to a conical receptacle and vice versa. For example, a conical shank of a drill matches the inside taper of a mating sleeve.
The connections serve a dual role: to align the axes of both the conical shank and conical receptacle and to provide stiffness to the connection. Since it is impossible to make conical surfaces with perfect accuracy, there are established standard classes of deviations for both the conical shank and receptacle (AT0, AT4, etc. with the tolerance becoming more loose with the increasing class number). In one arrangement, in all tolerance classes, the angle of the conical receptacle can be only smaller than the nominal angle, and the angle of the conical shank can be only larger than the nominal. Such tolerance allocation ensures that the conical surfaces of the shank and receptacle are in contact at the frontal (wider) portion of the connection thus providing a higher stiffness. However, it does not guarantee that the conical surfaces are in contact at their rear (narrower) end. Since standard tolerance allocations typically specify a negative deviation of the conical receptacle and a positive deviation of the conical shank, a clearance typically results at the rear end of the connection. For example, a typical AT4 quality has 13 angular second tolerance on each angle, which may result in radial clearance as high as 0.013 mm. at the rear end of the connection. This leads to mobility of the conical shank under heavy cutting forces and to significant runout if the drawbar force is not perfectly symmetrical. In another arrangement, the tolerance classes can be reversed with the angle of the conical receptacle being only larger than a nominal angle, and the angle of the shank being only smaller than nominal. Consequently, this tolerance allocation would ensure that conical surfaces of the shank and receptacle are in contact at the rear (narrower) portion of the connection. However, it would not guarantee that the conical surfaces are in contact at their front (wider) end.
A common application of conical connections is for connecting tool holders and spindles in machine tools. The term "tool holder" embraces, within the scope of the invention, all components which are used for performing machining or measuring operations and for this purpose are detachably fixed in a rotating or stationary spindle or in a stationary block. A tool holder typically includes a tapered shank that is adapted to be inserted into a tapered receiving bore of the spindle/block.
The shank-spindle interface is very important for both stiffness and accuracy in machining. Radial clearance at either end of the tapered connection due to taper tolerancing necessarily reduces stiffness and increases runout, and can cause fretting corrosion and fast wear of the spindle tapered receiving bore and will reduce machining accuracy.
Past attempts have been made to provide a high precision taper connection between the shank of the tool holder and the spindle that has both high radial and axial stiffness to prevent runout of the tool holder and increase machining accuracy. Yet, many of these arrangements require a complete redesign of existing tool holders and spindles and may be prohibitively expensive.
My goal in this invention was to develop an improved connection between a conical shank and a conical receptacle that overcomes many of the shortcomings of the prior art. I have found that my connection exhibits higher stiffness and better accuracy than conventional interfaces and is compatible with existing tool holders and spindles. Although my invention finds particularly important use in the connections between tool holders and spindles, it has many other applications.